Coated lamp



June 18, 1963 L. E. DUVAL 3,094,642

COATED LAMP Filed June 17, 1960 LEO E. DUVAL INVENTOR.

BY M- ATTO. NEY

United States Patent f 3,ii94,642 CGATED LAMP Leo E. Duval, Marblehead,Mass, assignor to Sylvania Electric Products Inc, a corporation ofDelaware- Filed June 17, 1960, Ser. No. 36,891 4 Claims. (Cl. 313-223)This invention relates to a protective coating for use on the exteriorsurface of an incandescent lamp. More specifically, this inventionrelates to a coating to be applied to a high silica content glassenvelope, which finds particular use in an incandescent lamp having highluminous efficiency. In particular, this coating finds application in aniodinetype incandescent lamp.

An iodine lamp is an incandescent lamp having high luminous efliciencyand a long useful life. Lamps of this type have an efiiciency oftengreater than 20 lumens per watt and a substantially better maintenancethan the prior incandescent lamp. The length of service is in the orderof hundreds of hours and possibly in excess of 2000 hours before anyappreciable darkening on the interior surface of the bulb takes place.

It has been determined that high efficiency for a long useful life canbe obtained by departing from the conventional aspects of comparativelylarge bulb size for a given energy dissipation and deliberatelyemploying a small compact glass envelope which contains an atmosphere ofiodine vapor. The iodine functions as a regenerative getter inassociation with the tungsten filament by combining with the evaporatedmetallic tungsten at the wall of the glass envelope to form tungsteniodide. This tungsten iodide then migrates to the vicinity of the hottungsten filament Where it is dissociated and the tungsten is returnedto the filament. It has been determined that this small compact bulbsize with high energy loading is necessary to maintain a comparativelyshort path for travel of the iodine vapor from the filament to the bulbWall to thereby minimize the possibility of recombination of thedissociated iodine before it migrates back to the bulb wall. Further,the compact size is also necessary to maintain the bulb wall attemperatures above a critical value to effect a reaction of the iodinewith the vaporized tungsten particles. It has been determined that foriodine to properly perform this function the inside bulb walltemperature should be maintained at approximately 250 C. minimum and amaximum of about 1200 C. to assure at the lower limit that the reactionwhich forms the tungsten iodide precedes at a proper rate and at theupper limit to avoid dissociation of the tungsten iodide. It is thusapparent that to obtain high efliciency in the lamp, relatively hightemperatures must be maintained at the bulb wall surface. Therefore,ordinary glasses such as those conventionally used in incandescent lampsare not suitable for use in this glass envelope. It is therefore generalpractice in the art to use a quartz glass or one having above about 96%fused silica.

Although each of these high silica content glasses are quite suitablefor use at the high temperatures, it has been determined thatsubstantial devitrification of the glass takes place when the lamps arehandled. This handling usually takes place either in the factory, in thepackaging operation or by the consumer when inserting the lamp into thefixture.

3,094,642 Patented June 18, 1963 ice Devitrification of glass is achange of the vitreous substance to a crystalline condition. That is,the formation of crystals in the glass. Devitrification tends to makethe glass translucent and brittle. It is apparent that to produce amarketable lamp, no devitrification should be allowed to occur in theenvelope.

The devitrification of the lamp envelope has been found to be due to itshandling prior to use. It would appear that sweat and oil present on thehands diffuse into the glass when the lamp is placed in operation at thehigh temperatures and causes the devitrification. In the packaging operation now being used extreme care must be taken to avoidhandling ofthe lamp. After the lamp has been purchased by consumers, they willinsert it into fixtures. These fixtures are designed to reflect themaximum amount of light very often to a fairly small area, and often theinsertion of the lamp may be rather difficult due to the positioning ofthe electrical connections. Examples of such fixtures are shown in thecopending application of David R. Dayton and David N. Brooks, entitledMovie Light, Serial No. 26,228, filed May 2, 1960.

I have now discovered that if the lamp is coated with an evanescentbarrier layer of plastic organic compound prior to packaging, thedevitrification will not take place in the envelope upon use even if itis handled. For purposes of this invention, an evanescent barrier layerincludes organic compounds which will be completely combusted when thelamp is placed in use. Also, the evanescent layer includes thosecompounds which will be vaporized from the surface of the lamp withoutcombustion such as by depolymerization and volitization of the monomer.It is apparent in all of these coatings that it is critical that therebe no carbonaceous deposits or residues remain on the surface of theenvelope after it has been placed in operation for a while, since suchresidues would readily reduce the luminous efficiency of the lamp.

It is an object of this invention to provide a means whereby a highluminous efiiciency lamp will not devitrify in use even if handled.

It is a further object of this invention to fabricate a lamp having asilica glass envelope and generating substantial heat which will not besubjected to devitrification even if handled.

Another object of this invention is to provide a coating 'of anevanescent barrier layer of an organic compound on the exterior surfaceof the bulb wall which will be gasified after the initial use by theconsumer.

A feature of this invention is that a lamp is produced which will not besubject to devitrification due to handling.

Other objects, features and advantages of this invention become apparentto those skilled in the art upon reading the following specification andthe accompanying drawings.

FIGURE 1 is a showing of the iodine lamp prepared according to thisinvention, inserted in an appropriate fixture.

FIGURE 2 is a cross sectional view taken along the lines 2-2 of FIGURE 3which shows in greater detail the iodine lamp.

FIGURE 3 is a showing along the lines 33- of FIG- URE 2 of theincandescent lamp'prepared according to this invention.

Referring now to FIGURE 1 of the drawing, the fixture comprises thesupport member 11 and the reflecting surface 13. Positioned within thereflecting surface 13 is the incandescent lamp 1. The reflecting surface13 is a concave, mirror-like surface. On either side of the reflectingsurfaces 13, small passages are provided so that the electrical contact17 may be attached to the incandescent lamp. In practice, it has beenfound advisable to make the support surface 11 and the reflectivesurface 13 a unitary body. Screws may be afixed in appropriate positionsto support members so that the reflector may be secured thereto.

Referring now to FIGURE 2, the sealed ends 9 of the glass tube 6 supportthe support members 4 which in turn support the tungsten filament 5 ateither end. The support element 4 may be suitable metal such asmolybdenum, tungsten or platium, and is sealed in the glass envelope 6so that the interior of the lamp 1 is isolated from the atmosphere. Theinside of the glass tube 6 is filled with suitable inert gases such asneon, krypton or argon. Also included within the glass tube is a smallquantity of iodine. This quantity of iodine should be sufficient toefiect the regenerative getter action required for the operation of thelamp but it should be insufficient to materially aifect the lighttransmission. These gases are added through an evacuation tube, theresidue of which is shown at 7.

-As will be noted on FIGURE 3 the coating 8 surrounds the entire glasssurface of the envelope. 'This coating can be effectively laid down bydipping, spraying, bench painting or other appropriate means; however,dipping is preferred due to the insured uniform coat.

Suitable for use as the barrier layer of the organic composition is acoating of an organic lacquer which may be,

'for example, acrylic acid esters, cellulose esters, or vinyl resins.Suitable solvents for these organic lacquers may be, for example, ethylalcohol or acetone. Other materials suitable for use include polyvinylresins such as polyvinyl acetate, polyacrylate and polymethacrylates. Of

these polymethacrylates, I have found that methyl, butyl andisobutyl-methacrylates, copolymers and mixtures thereof are quitesuitable. Cellulose acetate is highly suitable for use as this barrierlayer. Acetone is the principal solvent of cellulose acetate since itpossesses a high evaporation rate. Substituted cellulose acetate andorganic compounds are also suitable for use such as cellulose acetatebutyrate. A combination of cellulose acetate and cellulose acetatebutyrate are also suitable for the desired purpose. Also suitable foruse are the styrenes such as polystyrene, the appropriate ratio would bea 90/ 10% by Weight solution of xylene and polystyrene). Epoxy resinsare quite suitable as the barrier layer. The reaction'product ofepichlorohydrin and bisphenol in a 95/5% by weight xylene-resin solutionis satisfactory. The polyamids such as nylon are also useful. Adequatedevitrification inhibitation can also be given by cellulose nitrate inamylaceate. In all of these possible organic compounds the layer must berelatively thin and transparent. Also suitable for use are the urethaneplastics, such as polyurethane, in a suitable solvent.

As specific examples of this invention the following are offered. Theseexamples are merely preferred embodiments of the invention and are notintended to be limitative upon the claims.

Example 1 of coating composition and were air dried at room temperature,resulting in a clear transparent lacquer coating; after drying the lampswere freely handled. Within a few minutes after the lamps wereilluminated the coating was burned off leaving the lamps clean with noapparent residue. After continued use of the lamp, no devitrificationappeared.

Example 2 Example 3 A batch of isobutylmeth-acrylate was preparedaccording to the following specifications: 37% by weight resin in about62 /2% by weight of acetone. The lamps were air dried after dipping intothis batch and were freely handled. After drying, a clear transparentlayer of lacquer is left on the lamp. Within a few minutes after thelamps are turned on, the lacquer is burned off and the lamps are cleanand with no apparent residue of carbonaceous material and also noapparent devitrification.

Example 4 A 75/25 mixture by weight of xylol and cellulose acetatebuytrate is prepared. The lamps are dipped into the batch of thismixture and air dried. No precautions are taken as to handling. Afterthe initial illumination, the evanescent barrier layer is gasified andthe envelope of the lamp is free of divitrification.

Example 5 A batch of by weight of acetone and 20% by weight of polyvinylacetate is prepared. The lamps are dipped into the batch of coatingmaterial and air dried at room temperature. A clear layer of the resinis pro cluced after air drying. The lamps are handled freely and, afterthe initial illumination, no devitrification appears.

It is apparent that modifications may be made by those skilled in theart, my intent being only to be limited by the scope of the appendedclaims.

As my invention I claim:

1. An incandescent lamp having a glass envelope containing fused silica,said envelope being sealed from the atmosphere at the ends thereof,lead-in conductors sealed in said ends, a tungsten filament in saidenvelope, the ends of said filament connected to said lead-inconductors, a filling of an inert gas and a quantity of iodine in saidenvelope, said iodine being in quantities less than that which resultsin appreciable adsorption of light, but suflicient to effect aregeneration getter action, a layer of an evanescent organic compound onthe exterior surface of said lamp, whereby devitrification of the glassenvelope, when placed in service, is inhibited.

2. An incandescent lamp having an envelope prepared of a glass selectedfrom the group consisting of quartz and one having greater than 96%fused silica, said envelope being sealed from the atmosphere at itsends, leadin conductors sealed in each of said ends, a tungsten filamentin said envelope and being connected at its respective ends to saidlead-in conductors, a filling of an inert gas and a quantity of iodinein said envelope, said iodine being in quantities less than that whichresults in appreciable adsorption of light, but sufiicient to effect aregeneration getter action, and a barrier layer of an evanescent organiccompound on the exterior surface of said glass whereby devitrificationof the glass is inhibited.

'3. An incandescent lamp having an envelope prepared of a glass selectedfrom the group consisting of quartz and one having greater than about96% fused silica, said envelope being sealed from the atmosphere at itsends, lead-in conductors sealed in each of said ends, a tungstenfilament positioned in said envelope and connected at its respectiveends to the lead-in conductors, a filling of an inert gas and a quantityof iodine in said envelope, said iodine being in quantities less thanthat which results in appreciable adsorption of light, but sufficient toefliect a regeneration getter action, and an evanescent barrier layer ofan organic compound consisting of carbon, hydrogen and oxygen on saidglass, whereby devitrification of the glass is inhibited.

4. An incandescent lamp adapted to be heated to high temperatures andhaving a glass envelope containing high quantities of silica, saidenvelope being sealed from the atmosphere at each of its ends, lead-inconductors sealed References Cited in the file of this patent UNITEDSTATES PATENTS 1,655,488 Wolff et al Ian. 10, 1928 2,599,644 Keukens eta1 June 10, 1952 2,620,598 Jobling-Purser et al Dec. 9, 1952 2,743,150Rudy Apr. 24, 1956 2,883,571 Fridrich et al Apr. 4, 1959

3. AN INCANDESCENT LAMP HAVING AN ENVELOPE PREPARED OF A GLASS SELECTEDFROM THE GROUP CONSISTING OF QUARTZ AND ONE HAVING GREATER THAN ABOUT96% FUSED SILICA, SAID ENVELOPE BEING SEALED FROM THE ATMOSPHERE AT ITSENDS, LEAD-IN CONDUCTORS SEALED IN EACH OF SAID ENDS, A TUNGSTENFILAMENT POSITIONED IN SAID ENVELOPE AND CONNECTED AT ITS RESPECTIVEENDS TO THE LEAD-IN CONDUCTORS, A FILLING OF AN INERT GAS AND A QUANTITYOF IODINE IN SAID ENVELOPE, SAID IODINE BEING IN QUANTITIES LESS THANTHAT WHICH RESULTS IN APPRECIABLE ADSORPTION OF LIGHT, BUT SUFFICIENT TOEFFECT A REGENERATION GETTER ACTON, AND AN EVANESCENT BARRIER LAYER OFAN ORGANIC COMPOUND CONSISTING OF CARBON, HYDROGEN AND OXYGEN ON SAIDGLASS, WHEREBY DEVITRIFICATION OF THE GLASS IS INHIBITED.